To facilitate communications in a wireless system or data network, it is desirable to provide a plurality of wireless access nodes among which communications can take place via wireless links, the nodes optionally communicating via one or more wired connection paths with a wired communications network. In such a wireless network, wireless terminals can communicate with the nodes also via wireless links. For clarity herein, the wireless links via which the wireless terminals communicate with the nodes are referred to as access links, and the wireless links for communications among the nodes are referred to as transit links.
Such a wireless network may be referred to as an ad hoc network, in that wireless nodes can be easily added to or moved within the network to suit particular wireless data communications needs at any particular time. For example, the nodes can be distributed within a geographical region or area within which wireless access services are to be provided, and the wireless terminals can communicate among themselves and/or with the network via the various nodes. The wireless terminals can have any of various forms, and the communicated signals can comprise any desired form of information. Such a wireless system conveniently operates in a packet communications mode.
By way of example, the wireless communications via the access and transit links can be in accordance with known standards, such as the IEEE 802.11 standard for wireless LAN (local area network) communications. Channels in different frequency bands can be used for the access and transit links; for example channels in the 2.4 GHz band (IEEE 802.11b) for the access links and channels in the 5.2 and 5.7 GHz bands (IEEE 802.11a) for the transit links. However, this need not be the case and the access and transit links can use other frequency bands and/or can both use the same frequency band.
On initialization and re-initialization of a node in such a wireless network, for example on power-up, after maintenance, after recovery from an internal fault, or after recovery from a long outage of network communications, the node must discover the identities of its neighbouring nodes in order to initiate wireless communications with them, in order to discover communications or routing paths that the node can use. In contrast to a wired network in which this can be done simply by a node sending a “hello” signal via each of its physical interfaces (wired links), in a wireless network the node may have only one physical interface (a wireless link) via which it may communicate with many other nodes. Accordingly, the discovery process for a wireless node is relatively more complicated.
Other aspects of discovering and selecting a routing path in a wireless network relate to factors such as signal strength or transmitted power level, interference, and, for packet data networks, packet delay.
More particularly, in a wireless network the data rate of a wireless connection between two nodes can be variable and proportional to a power level used by the transmitter of the sending node to send the data; a higher power corresponds to a higher data rate. In a packet data network subjected to unpredictable bursts of traffic, congestion and delay management strategies may rely on using the highest possible data rate.
However, in many wireless networks the overall system communication capacity is limited by the amount of interference that each node in the network encounters. While part of such interference may be from sources external to the wireless network, wireless communications of other nodes within the network are a significant, and often dominant, source of interference. This interference is increased with higher power signals transmitted from the nodes.
In a wired packet data network it may be desirable to use a routing protocol which attempts to minimize packet delay by finding a lowest cost path through the network for any particular packet. Applying the same strategy to a wireless network, a first node would attempt to send a packet (e.g. containing information received by this node from a terminal via an access link) via a transit link to a second node that is the closest node to an intended destination (e.g. another terminal) of the packet. With increasing distances between the two nodes, for example if the terminals are far apart, the first node must use an increasing transmit power level, contributing to increased interference for other nodes in the network.
Accordingly, there are conflicting desires to minimize transmission power levels in order to minimize interference for the nodes of the wireless network, and to maximize transmission power levels in order maximize data rates and to minimize a number of communication hops among the nodes (and hence packet delay) in the wireless network. If a node of the wireless network is not able to make decisions autonomously to resolve this conflict, it must coordinate its transmissions with other nodes in the wireless network. Such coordination introduces communications overhead and further interference, and so there is a further trade-off between the coordination overhead and the benefits of coordination.
It would be desirable to provide a method of selecting a communications path via nodes of a wireless network which facilitates resolving these conflicts.